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Versions: 00 01 02                                                      
Internet Engineering Task Force           Kretschmer-AT&T/Basso-AT&T
INTERNET DRAFT                            Civanlar-AT&T/Quackenbush-AT&T
File:draft-ietf-avt-rtp-mpeg2aac-02.txt   Snyder-AT&T
                                          July 20, 2001
                                          Expires: January 20, 2002



       RTP Payload Format for MPEG-2 and MPEG-4 AAC Streams


                         STATUS OF THIS MEMO

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.

Internet-Drafts are working documents of the Internet Engineering Task
Force (IETF), its areas, and its working groups.  Note that other
groups may also distribute working documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time.  It is inappropriate to use Internet- Drafts as reference
material or to cite them other than as "work in progress."

The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt

The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.



                                 Abstract

This document describes a payload format for transporting
MPEG-2/MPEG-4 AAC encoded data using RTP. MPEG-2/MPEG-4 AAC is a
recent standard from ISO/IEC [1] [2] [3] for coding multi-channel
audio data. This payload format increases the packet loss resilience
of AAC coded audio transport above that of 'RTP Payload Format for
MPEG1/MPEG2 Video (RFC 2250)' [7] by incorporating AAC properties into
the payload format.  Supported features comprise fragmentation,
interleaving, grouping, repair information and a predictability
vector. The MPEG-2/MPEG-4 AAC bitstream format is not backwards
compatible with other MPEG-2 audio formats (e.g. MP3).  Several
services provided by RTP are beneficial for MPEG-2/MPEG-4 AAC encoded
data transport over the Internet. Additionally, the use of RTP allows
for the synchronization of MPEG-2/MPEG-4 AAC with other real-time
streams.

In this version of the draft:

         - The fragmentation section has been revised to allow for the
           fragmentation of elements (CPE, etc.) to better support
           transmission over small MTU links


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INTERNET-DRAFT     RTP Payload Format for MPEG AAC Streams     Jan 2002

1. Introduction

The ISO/IEC MPEG-2/MPEG-4 Advanced Audio Coding (AAC) [1] [2] [3]
technology delivers CD-like or better multichannel audio quality at
rates around 64 kBit/s per channel. It has a flexible bitstream syntax
that supports from 1 to 48 audio channels, up to 16 subwoofer channels
and up to 16 embedded data channels.  AAC supports a wide range of
sampling frequencies (from 16 kHz to 96 kHz) and an extremely wide
range of bitrates. AAC can support applications ranging from
professional or home theater sound systems to Internet music broadcast
systems.

The syntax of MPEG-2 AAC compressed data streams is identical to that
of MPEG-4 AAC main, AAC LC and AAC SSR General Audio compressed data
streams, so that MPEG-2 AAC is fully forward compatible with MPEG-4
AAC. Both MPEG-2 AAC and MPEG-4 AAC provide the same level of
compression performance. However, the semantics of MPEG-4 AAC is
different in one small respect, precluding a full backward
compatibility of MPEG-4 AAC to MPEG-2 AAC.

Benefits of using RTP for MPEG-2/MPEG-4 AAC stream transport include:

    i. Providing increased packet loss resilience based on application
    layer framing.

    ii. The ability to synchronize AAC streams with other RTP payloads

    iii. Monitoring MPEG-2/MPEG-4 AAC delivery performance through RTCP

    iv. Combining MPEG-2/MPEG-4 AAC and other real-time data streams
    received from multiple end-systems into a set of consolidated
    streams through RTP mixers

    v. Converting data types, etc. through the use of RTP translators.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [5].


1.1 Overview of MPEG-2/MPEG-4 AAC

AAC combines the coding efficiencies of a high resolution filter bank,
a powerful model of audio perception, backward-adaptive prediction,
joint channel coding, and Huffman coding to achieve high-quality
signal compression.  In 1998 the MPEG Audio subgroup tested the family
of MPEG audio coders (see http://www.tnt.uni-hannover.de/project/mpeg/
audio/public/w2006.pdf). The test results indicate that for a stereo
signal, AAC at 96 kBit/s has audio quality comparable to MPEG-2 Layer 3
("mp3") at 128 kBit/s.

AAC is a block oriented, variable rate coding algorithm.  An AAC
encoder takes 1024 samples per channel at a time (a 'block') as input
and the compressed representation is variable in size.

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Rate control can be used at the encoder to generate a constant-rate
bitstream. Each block of AAC compressed bits is called a "raw data
block", and can be decoded "stand-alone", that is, without information
from prior raw data blocks. This feature is particularly useful for
the delivery of AAC over lossy packet networks since the loss of a
packet does not directly affect the decodability of the adjacent
packets.


1.2 Bitstream Syntax

The syntax of an AAC bitstream is as follows:

<bitstream>        => <raw_data_block><bitstream>
<raw_data_block>   => [<element>]<END><PAD>

where <bitstream> indicates the AAC bitstream, <lowercase> indicates
intermediate tokens, <UPPERCASE> indicates terminal tokens and []
indicates one or more occurrence. <END> is a token that indicates the
end of a raw_data_block and <PAD> is a variable length token that
forces the total length of a raw_data_block to be an integral number
of bytes. In general, intermediate tokens are not an integral number of
bytes in length.

The <element> tokens are a string of bits of variable length, and they
can be any of the following:

<single_channel_element>     a single audio channel
<channel_pair_element>       a stereo presentation (2 channels)
<coupling_channel_element>   a mechanism for multi-channel compression
<lfe_channel_element>        a special effects channel
<data_stream_element>        "user data"
<program_config_element>     a mechanism for describing the bitstream
                             content
<fill_element>               a mechanism to use bits (for constant rate
                             channels)

The <elements> can occur several times in a single
raw_data_block. For example, the raw_data_block for a 5.1 surround
sound signal would be:

<single_channel_element><channel_pair_element>...
<channel_pair_element><lfe_channel_element><END>

corresponding to the center, left and right, left surround and right
surround and effects channels. Occurances of the <channel_pair_element>
are dis-ambiguated by means of a unique 4-bit id inside the
<channel_pair_element>.







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2. Issues covered by this Payload Format

2.1 Repair Information to reconstruct lost AAC Frames (Unequal FEC)

A smart AAC decoder can mitigate the effects of lost packets using
techniques such as interpolation in the spectral domain. However if
the raw_data_block in a packet is perceptually significant and also
highly unpredictable (e.g. the onset of a cymbal crash) then the
sender may choose to add repair information associated with that
raw_data_block. This form of unequal FEC allows the encoder/sender to
protect a stream depending on known loss characteristics and/or frame
predictability. A given repair information block (AAC DATA chunk with
TYPE > 0) is typically associated with a raw_data_block (TYPE = 0).
The association between the raw_data_block and the repair information
is obtained by means of the SEQ field.

Repair Information as defined here is a valid AAC raw_data_block.  As
an example, the Repair Information can be a highly compressed
monophonic version of a subset of the signal being transmitted. An AAC
stereo signal coded at a sampling rate of 44100 samples/s and a bit
rate of 96 kBit/s corresponds to an average raw_data_block size of 279
bytes.  A RepairData version of that block, compressed to 16 kBit/s
would be 46 bytes in length.  Given that perceptually critical blocks
might occur only once per 100 or more blocks, the average rate
increase associated with this type of RepairData can be very
low. Generally, the Repair Information for a given AAC frame X SHALL
be carried by a different RTP packet then the one that carries
X. Generally, the Repair Information MUST be computed at the same
sampling rate as the stream being repaired.

The usage of the Repair Information is similar to the one proposed
in [6].  The OPTIONAL Repair Information MAY be provided for every
frame.  RepairData can be generated in many ways including using two
encoders, decoding followed by coding or processing the original
bitstream.


2.2 Fragmentation of AAC Frames

It is desirable to limit the size of an AAC frame to less than the
path-MTU. If this is not possible, the frame can be fragmented across
several RTP packets. Fragmentation SHOULD occur at <element> boundaries.
If further fragmentation is needed <elements> MAY be fragmented, as well.
In that case the decoder must be able to handle partial <elements>.

An RTP packet contains either an integer number of complete AAC frames
or fragments of a single AAC frame. Subsequent RTP packets containing
subsequent fragments of an AAC frame have a much simpler header that
is just two bytes long. They can be identified by the F-bit set to
1. The S-bit signals the first or only fragment of an <element>. The
same ELEMENT ID is shared by all fragments belonging to the same
<element>. ELEMENT ID zero is assigned to the fragments of the first
<element> in the frame and is increased by one for each following
<element>. FBITS indicates the number of unused bits in the first byte

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and LBITS the number of unused bits in the last byte. FBITS and/or
LBITS must be set to zero if the fragment starts and/or ends at a
byte-aligned boundary. The length of a fragment can be determined from
the total length of the packet excluding the headers.

0                   1                   2                   3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|X|X|X|F|S|ELEMNT ID|FBITS|LBITS|AAC FRAGMENT                   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


2.3 Predictability of AAC Frames

AAC frame predictability allows adaptive handling of packet losses
and/or bandwidth constraints by signalling the need for an action a
receiver may take when an associated AAC frame is lost. Every AAC frame
will be assigned to one of the following three predictability classes:
 - 0: not predictable
 - 1: one side predictable (either L-predictable or R-predictable)
 - 2: two side predictable
(- 3: reserved)

An AAC frame that belongs to class 0 cannot be easily concealed using
any other AAC frame(s) in the bitstream.

An AAC frame that belongs to class 1 can be predicted either from
previous (R-predictable) or following (L-predictable) AAC frame but
not from both.

An AAC frame that belongs to class 2 can be predicted from the
preceding or following AAC frame or from both.

Predictability information is coded for every RTP AAC packet in the
Predictability Quantifier (PQ) which is 2 bits in length. For a given
RTP packet such PQs are organized in a predictability vector which
represents a sliding window of PQs, starting with the current packet's
PQ followed by preceding packets' PQs.


2.4 Grouping and Interleaving of AAC Frames

It is often desirable to group an integer number of AAC frames. The
predictability of such an RTP packet is the predictability of the AAC
frame in the RTP packet which is least predictable. AAC frames
belonging to the same predictability class MAY be grouped into one RTP
packet. Note that if frames of different predictabilities are grouped
much of the usefulness of the predictability information is lost. The
sequence numbers SEQ of the AAC DATA chunks are used to restore the
proper order on the receiver side.

Grouping AAC frames into a single RTP packet is OPTIONAL.

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2.5 Example RTP Packet Sequence

The example below shows a sequence of AAC frames (a...p) and their
assigned predictability classes.

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| a | b | c | d | e | f | g | h | i | j | k | l | m | n | o | p |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 2 | 2 | 2 | 1 | 0 | 1 | 2 | 2 | 2 | 2 | 2 | 1 | 0 | 1 | 2 | 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The AAC frames MAY be grouped according to their predictability.
R(x) is the RepairData information sent within the RTP packet:

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|a g j|b h k|c i o| d f |  e  | l n |  m  |  p  |           |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
|     |     |R(e) |     |     |R(m) |     |     |     |     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


3. RTP AAC Payload Format

The AAC specific RTP payload consists of a 8, 32, 64 or 96 bit header,
and a variable number of AAC DATA chunks. The type of those chunks is
identified by TYPE field. The LENGTH field specifies the length of a
chunk in bytes and SEQ is a sequence number which allows grouping,
interleaving and association of Repair Info with the frame it repairs.

The header contains a vector of Predictability Quantizers (PQ) which
specify the packets' predictability classes, and a set of control
bits.

The PVS field specifies if the header contains 12, 28 or 44 PQs.  At
the beginning of a session, if fewer packets have been transmitted/
received than there are PQs in the header then the extra PQs are
invalid and MUST be set to 0 (on the sender side) and MUST be ignored
(on the receiver side).

If a sender provides a predictability vector but does not provide
frame predictability information it MUST set all PQs to 0. A client
can ignore the information provided by PQs since PQs are not required
for decoding AAC frames. PQs can be used to decide when to ask for
retransmission of lost packets. PQs can also provide hints which help
a PQ-aware decoder to improve the audio quality when concealing lost
packets.









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0                   1                   2                   3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|PVS|M|F|  MBZ  |PRD VECTOR (PVS > 0)                           | Header
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|PRD VECTOR (PVS > 1)                                           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|PRD VECTOR (PVS > 2)                                           |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
|TYPE   |SEQ                    |LENGTH (if M==1)               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
|AAC DATA 1                                                     | AAC
|                                                               | Data
|                                                               |
|               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|               |TYPE   |SEQ                    |LENGTH         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
|               |AAC DATA 2                                     |
|-+-+-+-+-+-+-+-+                                               |
|                               .                               |
|                               .                               |
|                               .                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
|TYPE   |SEQ                    |LENGTH                         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
|AAC DATA N                                                     |
|                                                               |
|                                                               |
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


PRD VECTOR: Predictability vector. It contains either 12, 28 or 44
            Predictability Quantifiers (PQ). The size of a PQ element
            is 2 bits. The first PQ refers to the predictability class of
            the current packet. The following PQs refer to the most
            recent previous packets. Thus, the vector looks like this:
            {PQ(t), PQ(t-1), PQ(t-2)...}
            The predictability class of a packet is that of the least
            predictable AAC frame that is contained in the packet.

PVS:        Predictability Vector Size. Specifies the number of 32bit
            words used for the Predictability Vector. The first 32bit
            word contains the flags field. Hence, only the lower
            significant 24bits belong to the vector. If PVS is set to
            0 the predictability vector field does not exist, and
            the TYPE field is contiguous with the MBZ field.

M:          If M is set, then the payload contains more than one AAC
            frame. Hence, a LENGTH field for the first frame MUST be
            present. If M is clear, then only one AAC frame is present,
            and no LENGTH field is present.



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F:          Fragmented Frame.


MBZ:        Must be set to 0.

TYPE:       The type of AAC DATA. This field specifies if the AAC DATA
            is an original AAC frame or contains some form of Error
            Correction data. The following types are defined for now:
            0: Original AAC frame
            1: Identical to original frame but sent for redundancy
            2: Same AAC configuration but encoded using less bits
               (Repair Information)
            (all three types are valid AAC frames)

SEQ:        The sequence number enumerates AAC frames at the stream level.
            It may be used to support interleaving or association of Repair
            Information with TYPE==0 AAC frames, etc.

LENGTH:     The length of the AAC Data in bytes.

AAC DATA:   The actual AAC data chunk. This is either a valid AAC frame
            (TYPE = 0) or Repair Information belonging to a valid AAC frame
            (TYPE > 0).


3.1 RTP Header Fields Usage:

The RTP header fields are used as follows:

Payload Type (PT): It is expected that the RTP profile for a
particular class of applications will assign a payload type for this
encoding, or alternatively a payload type in the dynamic range shall
be chosen.

Marker (M) bit: Set to one to mark the last fragment (or only
fragment) of an AAC frame.

Extension (X) bit: Defined by the RTP profile used.

Timestamp (TS): 32-bit timestamp representing the sampling time of the
first sample of the first AAC frame in the packet. The clock frequency
MUST be set to the sample rate of the encoded audio data and is
conveyed out-of-band (i.e. through SDP [8]). If N > 1 frames are
present in a RTP packet the TS of the frames 2...N can be calculated
by computing the sequence number difference between those frames and
the first frame since the sample rate and the number of samples per
frame are fixed and known. All packets that make up a fragmented AAC
frame MUST use the same TS. Timestamps start at a random value to
improve security.

SSRC: set as described in RFC1889 [2].

CC and CSRC fields are used as described in RFC 1889 [2].


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RTCP SHOULD be used as defined in RFC 1889 [2]


4. Security Considerations

RTP packets using the payload format defined in this specification are
subject to the security considerations discussed in the RTP
specification [2]. This implies that confidentiality of the media
streams is achieved by encryption. Because the data compression used
with this payload format is applied end-to-end, encryption may be
performed on the compressed data so there is no conflict between the
two operations.


This payload type does not exhibit any significant non-uniformity in
the receiver side computational complexity for packet processing to
cause a potential denial-of-service threat.


5. Intellectual Property Disclosure

A US patent application has been filed on the usage and computation
of predictability information for transmission over lossy channels.


6. References

  [1] ISO/IEC 13818-7 Advanced Audio Coding (AAC).

  [2] ISO/IEC 14496-3:1999 "Information technology - Coding of
  audio-visual objects - Part 3: Audio," December, 1999.

  [3] ISO/IEC 14496-3:1999 / AMD1:2000.

  [4] Schulzrinne, Casner, Frederick, Jacobson RTP: A
  Transport Protocol for Real Time Applications  RFC 1889,
  Internet Engineering Task Force, January 1996.

  [5] S. Bradner, Key words for use in RFCs to Indicate
  Requirement Levels, RFC 2119, March 1997.

  [6] Perkins C., Kouvelas I., Hodson O., Hardman V., Bolot J.C,
  Vega-Garcia A., Fosse-Parisis S. "RTP Payload for Redundant Audio Data",
  RFC 2198, Internet Engineering Task Force, September 1997.

  [7] D. Hoffman, G. Fernando, V. Goyal, M. Civanlar
  RTP Payload Format for MPEG1/MPEG2 Video  RFC 2250,
  Internet Engineering Task Force, January 1998.

  [8] M. Handley, V. Jacobson, SDP: Session Description Protocol
  RFC 2327, Internet Engineering Task Force, April 1998.




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7. Authors' Addresses

Mathias Kretschmer
AT&T Labs - Research
180 Park Ave.
Florham Park, NJ 07932
USA
e-mail: mathias@research.att.com

Andrea Basso
AT&T Labs - Research
100 Schultz Drive
Red Bank, NJ 07701
USA
e-mail: basso@research.att.com

M. Reha Civanlar
AT&T Labs - Research
100 Schultz Drive
Red Bank, NJ 07701
USA
e-mail: civanlar@research.att.com

Schuyler R. Quackenbush
AT&T Labs - Research
180 Park Ave.
Florham Park, NJ 07932
USA
e-mail: srq@research.att.com

James H. Snyder
AT&T Labs - Research
180 Park Ave.
Florham Park, NJ 07932
USA
e-mail: jhs@research.att.com



















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